The Newtonian telescope is named after its inventor, Sir Isaac Newton. A typical Newtonian telescope is schematically illustrated in FIG. 1. In the conventional Newtonian design, light is collected by a parabolic primary mirror 10 which is positioned in a telescope tube 12 opposite an opening 14 which is pointed at the sky. The parabolic surface of the primary mirror reflects collected light back up the tube along the optical axis of the primary toward a focal plane, the position of which is determined by the diameter and focal ratio of the primary mirror. Before the converging reflected light beam reaches a focal or image plane however, the beam is reflected at a substantially 90° angle toward and through the telescope tube by a secondary mirror 16 or less commonly by a prism. The Newtonian secondary mirror 16 is also commonly known as a diagonal or diagonal mirror. The secondary mirror or prism is positioned with respect to the primary mirror at a carefully selected distance so that the focal plane may be conveniently accessed with an ocular or camera associated with a focuser 18 on the outside of the telescope tube.
The secondary mirror in a classical Newtonian design has a precisely planar front surface which faces the primary mirror. This front surface is aluminized or otherwise made highly reflective. Secondary mirrors typically are ellipsoid in shape having a major axis and minor axis. The tilted elliptical shape of the secondary mirror provides a circular profile to incoming light rays. The substantially circular elliptical shadow has a diameter substantially equal to the width of the minor axis of the ellipse. Thus, an elliptical mirror may maximize the reflective surface area presented to the converging light beam while minimizing the actual obstruction placed into the path of incoming light rays.
As shown in FIG. 2, the secondary mirror 16 is typically supported by a secondary mirror mount 19. Referring back to FIG. 1, it may be noted that the secondary mirror mount is usually supported by a spider assembly 20 which may include one or more straight or curved vanes or legs which suspend the secondary mirror mount at the appropriate place inside the telescope tube. The proper functioning of a Newtonian telescope is dependent upon the effective collimation of each optical element such that the focal plane produced by the optics is positioned accurately with respect to the focuser and not unduly tilted with respect to the optical axis of an ocular or the image plane of a camera connected to the focuser. To achieve proper collimation, both the primary mirror mount and the secondary mirror mount may incorporate various adjustment apparatus.
To achieve proper Newtonian collimation at the secondary mirror, at least three types of alignment criteria must be observed. In particular, the lateral positioning of the secondary mirror in a plane perpendicular to the optical axis of the primary mirror must be adjusted so that the center point of the secondary mirror is substantially on the optical axis of the primary, or offset slightly away from the optical axis of the primary mirror to achieve certain advanced optical performance goals. Secondarily, the positioning of the secondary mirror along a line parallel to the optical axis may be adjusted and offset as well to achieve similar performance goals. Thirdly, the angular alignment of the secondary mirror with respect to the primary optical axis must be adjusted to make the optical axis of the primary mirror and the optical axis of a selected ocular coincide.
The first and second types of adjustment described above are of relatively minor importance when compared to the angular alignment adjustment. For example, failure to achieve perfect lateral alignment or perfect positioning along the optical axis of the primary will result in the secondary mirror not being placed precisely at the center of the primary optical axis. This defect will be unnoticed if the secondary is slightly oversized. Non-centering error may result in uneven field illumination or vignetting but will not otherwise introduce optical aberrations into the system provided that accurate angular alignment is maintained. On the other hand, failure to properly angularly align the secondary mirror with the optical axis of the primary will cause the image plane to be substantially tilted with respect to the optical axis of an ocular or a camera image plane. This error will both enhance aberrations such as coma which are inherent in the Newtonian design and introduce optical aberrations throughout the image plane which are not otherwise inherent in the system.
Most known Newtonian secondary mounts include apparatus for adjusting the angular alignment of the secondary mirror. For example, as shown in FIG. 2, many secondary mirror mounts include three or four adjustment screws 22 which articulate a joint between a secondary mirror holder 24 and a mount body 26 which is attached with a bolt or stud 28 to the spider assembly. The prior art embodiment shown in FIG. 2 has four adjustment screws. Each of these screws acts along a line parallel to the optical axis of the primary. Two of the screws, 22a and 22b, articulate the secondary mirror about axis x-x which is substantially orthogonal to the primary optical axis. It will be readily noted that axis x-x is also parallel to (although offset quite far from) the minor axis of the secondary mirror. The remaining two adjustment screws 22c and 22d however, articulate the secondary mirror about the axis shown on FIG. 2 as y-y. Axis y-y is not parallel to the secondary mirror plane.
The angular alignment of a secondary mirror with respect to the optical axis of the primary mirror may be visualized as having two components. The major axis of the secondary must be positioned to intercept the converging light cone at a 45° angle (assuming that the focuser is positioned at a 90° angle with respect to the primary axis, which is a nearly universal design goal) while the minor axis of the secondary mirror remains orthogonal to the primary optical axis. Since secondary adjustment screws 22c and 22d define an adjustment axis which is not parallel to a plane defined by the face of the secondary mirror, adjustment about this axis effects both the angular orientation of the major axis and the orthogonality of the minor axis. Therefore adjustments which are made in a conventional design about axis y-y interact with previous or future adjustments made about axis x-x. Accordingly, the proper angular collimation of a Newtonian secondary using a conventional secondary mirror mount becomes a tedious reiterative process which may be difficult to perform correctly.
In addition, adjustment mechanisms which articulate at a distance relatively far from the mirror face as shown in FIG. 2 necessarily result in a relatively large amount of side to side or fore-aft translation motion or swing of the entire mirror when adjustments are made. Thus, angular adjustments made with a conventional secondary holder as shown in FIG. 2 may complicate attempts to accurately place the secondary mirror laterally or along the primary optical axis. The commonly available alternative secondary mirror holders which have three independent alignment screws acting parallel to the primary axis define three separate adjustment axes only one of which can potentially be parallel with the plane of the secondary mirror. Thus, adjustments made to the angular orientation of the secondary mirror with any of three angular adjustment screws will also interact with the adjustments made with other angular adjustment screws.
The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.